It's supposed to be the "gold standard" of evidence supporting the standard model of cosmology - including dark matter, dark energy and the exponential expansion after the big bang known as inflation.

But could it be wrong? Might misleading measurements by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) have been leading us towards the wrong theory of cosmology? One astrophysicist thinks so, and he says the planet Jupiter is to blame - though others insist that there is nothing amiss.

WMAP detects photons of the cosmic microwave background, the "echo" of the big bang, and these measurements are used to map the temperature of the sky. Ripples in the map are used to calculate a spectrum that produces a near-perfect fit to the standard model of cosmology.

Since 2007, Tom Shanks at the University of Durham, UK, who is a critic of the standard model, has been tracking a discrepancy between measurements from WMAP and X-ray measurements of some of the same star clusters made by ground-based telescopes. He initially assumed that the problem was with measurements from the ground and that the WMAP data was "pristine".

To calibrate the microwave data, WMAP scientists use the planet Jupiter, which they assume to be a steady source of microwaves. Observations of Jupiter show how much the cosmic ripples are being blurred by the instrument's optics, allowing the WMAP team to correct for this.

Now Shanks and his PhD student Utane Sawangwit have recalibrated the data using objects such as radio galaxies observed by WMAP that also emit microwaves. The result is a spectrum that is compatible with a host of theories that the WMAP team claims to have ruled out. For example, one-dimensional cosmic strings - defects in the fabric of space-time - or modified laws of gravity might explain the clumping of matter that is currently attributed to the dark matter and dark energy of the standard model. "If we're right, that would be incredibly important for cosmology," says Shanks.

However, he adds that it remains unclear why Jupiter would produce an unreliable calibration. "That problem is not understood as yet," he says. The paper (arxiv.org/abs/0912.0524) will appear in Monthly Notices of the Royal Astronomical Society: Letters.

The WMAP team isn't taking the challenge lying down. They claim that the radio sources observed by WMAP coincide with spots of the sky where the temperature is slightly higher, making the calibration inaccurate. "We're happy to defend WMAP," says team member Gary Hinshaw of NASA Goddard Space Flight Center in Greenbelt, Maryland.